1. Field of the Invention
The present invention relates to a carrier preferably used for electrophotography (xerography), electrostatic recording method, electrostatic printing method, and the like and also relates to a method for producing the carrier, a developer and an image forming method using the developer.
2. Description of the Related Art
Dry developing method used for electrophotography is a method in which a toner frictioned with a charting unit is made to electrostatically adhere on a latent electrostatic image to form a visible image.
The dry developing method is divided into a so-called one-component developing process using primarily a toner, and a so-called two-component developing process using glass beads, and a magnetic carrier, or using a mixture of a coat carrier with the surface thereof coated with a resin, and a toner.
A developer used for a two-component developing process is held by electric force generated from friction between microscopic toner particles on the surface of relatively large core material, and when the developer is made to closely contact with a latent electrostatic image, the electric field formed by the latent electrostatic image makes the toner particles superior to the binding force between the core material and the toner particles to thereby develop the toner on the latent electrostatic image. Then, the developer is repeatedly used while replacing a toner that has been consumed in developing.
For the reason, during the use of a toner for long hours, the toner material must be frictionally charged with a polarity constantly requesting toner particles and a sufficient charge amount. However, inter-particle collision, mechanical agitation of particles in a developing unit, and/or heat generated therefrom make a toner fused on the core material surface, in other words, a so-called spent toner is caused on a core material surface, and the charge property of the core material is degraded with use time. As the result, background smear of image and toner scattering are caused, and thus it gives rise to a need to replace the developer entirely.
To prevent occurrences of such a toner spent, it is tried to make a carrier have a longer operating life by coating a surface of a core material with a resin having low surface energy, for example, a fluorine resin, a silicone resin, etc. For example, there are the following carriers. A carrier coated with a room temperature curable silicone resin and a positively charged nitrogen resin (Japanese Patent Application Laid Open (JP-A) No. 55-127569); a carrier coated with a coating material containing at least one modified silicone resin (Japanese Patent Application Laid Open (JP-A) No. 55-157751); a carrier having a coating layer containing a room temperature curable silicone resin and a styrene-acrylic resin (Japanese Patent Application Laid Open (JP-A) No. 56-140358); a carrier of which surfaces of nucleic particles are coated with two or more layers of silicone resins so as not to have adhesiveness between the layers (Japanese Patent Application Laid Open (JP-A) No. 57-96355); a carrier of which nucleic particle surfaces are coated with silicone resins in a multi-layered structure (Japanese Patent Application Laid Open (JP-A) No. 57-96356); a carrier of which carrier particle surfaces are coated with a silicone resin containing a silicon carbide (Japanese Patent Application Laid Open (JP-A) No. 58-207054); a positively charged carrier coated with a material exhibiting a critical surface tension of 20 dyn/cm or less (Japanese Patent Application Laid Open (JP-A) No. 61-110161); and a developer which contains a carrier coated with a coating material containing fluoroalkyl acrylate, and a toner containing a chrome azo dye (Japanese Patent Application Laid Open (JP-A) No. 62-273576).
In recent years, to obtain higher image quality, toner size tends to be made into smaller particle diameters, and as the result, a toner spent to a carrier in a two-component developer easily occurs. In addition, with a conventional spray coating, it is difficult to get the carrier surface uniformly wet with a coating material, and therefore it has become difficult to produce a carrier having appropriate adhesiveness between the coated film and the core material and uniform thickness and film quality. Further, in the case of a full-color toner, a low-softening point resin is used to obtain sufficient color tone, and therefore the full-color toner causes much more amount of toner spent to a carrier than in the use of a black toner, and this leads to reduction in charge amount of toner and also leads to toner scattering, and background smear. In a full-color electrophotographic system, when the charge amount of toner is reduced, the image density easily changes particularly at highlighted portions to make it unable to keep high-image quality.
Since there are microscopic concaves and convexes or irregularities on a carrier particle surface, the coated layer is thickened in concave portions, and convex portions have a thin coat layer or are not coated sufficiently, it may be difficult to make a coating layer into a dense, well-adhesive and uniformly formed thin layer and to coat a core material with such a thin layer. As the result, with the use of a conventional carrier, the toner charge ability and the temporal stability are poor, and this causes various problems such as background smear and abnormal image as well as toner spent to the carrier.
In addition, there are problems in manufacturing process such as restrictions of volatile organic compounds (VOC) generated from organic solvents used in forming a coating layer on the core material surface, occurrences of waste fluid, and a further necessity of dry energy. The conventional methods for producing a carrier and conventional carriers have may problems to solve, from the viewpoint of high-cost performance, global environmental burden, and natural resource saving.